Due to environmental considerations, exhaust after-treatment devices are often fitted to vehicles in order to reduce particulate matter and NOx emissions therefrom.
However, the sulphated ash, sulphur and phosphorus concentrations of lubricating oil compositions conventionally used in internal combustion engines may have adverse effects on such after-treatment devices.
Sulphated ash is the total weight percent of ash in a lubricating oil composition. The sulphated ash content in a lubricating oil composition is related to the total metal content therein. Sulphated ash may be conveniently measured according to ASTM D874.
In view of the adverse affects that the sulphated ash, sulphur and phosphorus concentrations of lubricating oil compositions may have on exhaust after-treatment devices, it is therefore prudent to develop lubricating oil compositions with reduced sulphated ash, sulphur and/or phosphorus concentrations therein.
Phosphorus concentrations may be generally reduced by reducing the amount of zinc dithiophosphate which is present as an anti-wear additive in lubricating oil compositions.
Sulphur levels in lubricating oil compositions may be reduced by employing low sulphur level base oils and reducing the amount of sulphur-containing additives employed therein.
The major sources of sulphated ash in a lubricating oil composition are generally any metal detergent additives and zinc dithiophosphate anti-wear additives that are employed therein.
Metal detergent additives perform several roles including acting as a cleaning agent on metal engine surfaces, neutralising acids and providing anti-oxidant properties to the lubricating oil composition.
However, reduction of the amount of metal detergent additives in a lubricating oil composition has an adverse effect on the detergency thereof.